Updraft traveling grate pelletizing furnace



March 6, 1962 L. J. ERCK ETAL 3,024,101

UPDRAFT TRAVELING GRATE PELLETIZING FURNACE Filed May 25, 1956 9Sheets-Sheet 1 IN VEN TORS 100/5 7'- EEC/T, DON/u a c. V/0Z7'7 4 a:THOMAS 5 BA 1v.

ATTORNEYS March 6, 1962 J. ERCK ETAL UPDRAFT TRAVELING GRATE PELLETIZINGFURNACE Filed May 25, 1956 March 6, 1962 I L. J. ERCK ETAL UPDRAFTTRAVELING GRATE PELLETIZING FURNACE Filed May 25, 1956 9 Sheets-Sheet 4vmw v v A S S TN. N Y o mnAm w m w 2 "n a N A 7 I I r J sw 5mm" wmwa wwr m H W R koQQhuW K WN March 6, 1962 J. ERCK ETAL UPDRAFT TRAVELINGGRATE PELLETIZING FURNACE Filed May 25, 1956 9 Sheets-Sheet 5 ll vlllllv m V, 5% W fl OCM E E /B N N MEV. C. E W? m 1 5)" 7 WW m March 6,1962 L. J. ERCK ETAL 3 UPDRAFT TRAVELING GRATE PELLETIZING FURNACE FiledMay 25, 1956 9 Sheets-Sheet s UFO/73A F7 c004 ING INVENTORS 400/5 .71EEC/f, DO/VAZD c. wozerm d3 THOMAS F. BA/v.,

IGNITION March 6, 1962 J. ERCK ETAL 3,024,111

UPDRAFT TRAVELING GRATE PELLETIZING FURNACE Filed May 25, 1956 9Sheets-Sheet '7 IN V EN TORS A 00/5 .7. EEC/v,

DON/1Z0 C- V/OLETM l l J By JHo/ws BAN.

March 6, 1962 J. ERCK ETAL UPDRAFT TRAVELING GRATE PELLETIZING FURNACEFiled May 25, 1956 9 Sheets-Sheet 9 GIE'A TE CONVEYOR A 5 m N wmA N m moE Vsv E r m 0 m 7 on A N N u o b O Hm G A o Yw Bl 9 W W m 2 mu 2 n 0 mPm we 7mm o N M s w F.

mmm mm H my 3,924,1fil Patented Mar. 6, 1962 3,024,101 UPDRAFT TRAVELINGGRATE PELLETIZING FURNACE Louis J. Erck, Negaunee, and Donald C.Violetta, Ishpeming, Mich, and Thomas E. Ban, Cleveland, Ohio, assignorsto The Cleveland-Clifis Iron Company, Cleveland, Ohio, a corporation ofOhio Filed May 25, 1956, Ser. No. 587,393 13 Claims. (Cl. 75-5) Thepresent invention relates generally to the art of iron-ore beneficiationand is more particularly concerned with a novel method of agglomeratingfinely-divided iron ores and with new apparatus implementing the saidnovel method.

The iron-ore mining industry in this country turned its attention someyears ago to the problem of beneficiating horizontal ores because theprincipal sources of high grade ores were being rapidly depleted. Thevirtually inexhaustible supply of lower grade iron formations averagingabout 30% to 40% iron in finely-textured rocks of the taconite andjasper types occurring in the Mesaba and Marquette ranges has offered aspecial incentive to the industry to develop some means for economicrecovery of the iron values of these formations. It has been estimatedthat marginal ores of this type aggregate in the Minnesota and Michiganiron-mining areas a total of 70,000,000,000' tons compared to anaggregate of about 4,000,000,000tons of the higher grade ore which wasoriginally present there.

The advantages to be gained through the development of a commerciallyfeasible means for upgrading marginal domestic ores consequently arespectacular. Furthermore, they are not limited to the interests owningor operating iron-ore mines and mining facilities in this country, butare important to the country as a Whole. The costs of bringing foreignores to this country for reduction are comparatively high, making thecost of iron and steel and everything made by or with themcorrespondingly greater. Moreover, national self-sufficiency as to themost important natural resource will be lost to this country if thesedomestic ore horizons cannot be successfully developed.

Since the upgrading of lean ores may, in theory, be carried to the pointthat relatively high grade iron oxide is produced, there is anadditional advantage of potentially great commercial significance to begained through the development of any feasible procedure forconcentrating these ores. Thus, instead of offering steel mills ores of51.5% iron (natural), it is a practical possi bility to furnish ores of66% iron (natural) for their furnaces. This Would materially reduce thecost of furnace operation because of the higher yields of metal, and itwould increase metal production facilities without the enlargement ofany existing equipment or addition of any new furnaces.

With the advantages to be gained being so great and with theconsequences of failure being so disastrous, there have naturally beenmany determined efforts made to develop such a process, and large sumsof money have been expended in this direction. As a result, thetechnology of iron ore beneficiation has been advanced long distances invarious directions and phases in the past fifteen years since theproblem entered its acute stage. However, only very recently have therebeen commercial operations involving marginal ores and prior to thepresent invention, these operations were limited to magnetic ores.

New techniques and tools have been devised to overcome the specialdifficulties in mining taconite and jasper. In actual practice, thesetechniques and apparatus have established themselves as satisfactory forcommercial use,

being effective, rapid and economical by comparison with currentopen-pit and shaft-mining operations.

Similarly, new techniques and machines have been developed forcommercial use in the extraction of the ore values from the gangues ofmarginal ores. The procedure of froth flotation has been refined, andchemical research has yielded frothing agents which have improved theyields in. flotation operations to the point where they are commerciallyacceptable. In addition, some new tools have been developed, and someolder ones have been adapted for the extraction of fine, specular ironoxides from mixtures of predominantly siliceous material. Thus, theso-called heavy-media process in current use predominantly on the Mesabirange may involve the use of the cyclone, the Hardinge separator and theAkins classifier.

As for the particular processes and procedures employed, those disclosedand claimed in copending application Serial No. 206,448, filed January17, 1951, now Patent No. 2,744,627, in the name of Louis I. Erck, givespecial promise of rapidly and consistently producing premium oreproducts and have the advantage of being usable in a conventionalHardinge machine. They also offer the important advantage of enablingthe use of relatively inexpensive materials as the separating media.Thus, ferrosilicon can be replaced with magnetite, under certaincircumstances, to improve the economy or to simplify the separatingprocess.

The finely-divided ore values recovered from low grade or horizontaldeposits cannot be charged into a commercial reduction furnace becausetheir particle size is typically minus mesh. Therefore, some type ofagglomerating operation is essential to their utility. Sinteringprocesses and conventional sintering machines have been employed invarious ways in a number of efforts to produce a satisfactoryagglomeratefrom these fines, but the results have been unsatisfactory tothe point Where the art generally has turned to pelletizing as a logicalapproach. The tendency for this ore to pack in a sintering operation issuch that normal operation of the sintering machine is not consistentlypossible. Furthermore, sintering machines are of low capacity so thattheir use would either be too expensive or time-consuming.

Prior to the present invention to the best of our knowledge, theproblems associated with the production from these ore fines of ballproducts suitable as furnace charge have not been totally overcome.These problems break down into two general categories according tothe.two general stages of the balling procedure, i.e., the forming of thegreen ball from the ore fines and the firing or burning of the greenball to provide it with the physical characteristics necessary to see itthrough the handling, shipping and charging operations incident to itsuse as furnace charge.

In the first stage, good results in large-scale operations have beenconsistently obtained through the use of the process and the apparatusdisclosed and claimed in our copending application Serial No. 580,423,filed February 15, 1956. Also, the process disclosed and claimed incopending application Serial No. 346,786, filed April 3, 1953, nowPatent No. 2,799,572, in the names of Grover J. Holt and Louis J. Erck,offers special advantages in this use. The combination of the teachingsof these two disclosures, furthermore, produces consistently superiorresults and represents a development in the art which stands out far inadvance of the field and apparently meets and disposes of all theproblems and difficulties in the prior art relating to the production ofgreen balls of good physical characteristics.

Accordingly, the final link in the chain of this development is thefiring of the pellets, and it is at this obstacle that the art hasarrived and has been impeded in its progress for some time. These haveall had important shortcomings and derelictions which have beengenerally recognized and tolerated in commercial operations because theyrepresented the best that the art afforded. In particular, theconventional shaft furnace or kiln has inherent operating deficienciesand traveling grate-type furnaces operating on the down-draft principlealso have restricted capacity and relatively high thermal requirements.Also, grate bar maintenance is a significant cost factor in these latterfurnaces due to the high temperatures to which the grates are subjectedin pelletizing operations, i.e., 2000" F. or higher.

In accordance with the present invention, it is, for the rst time, tothe best of our knowledge, possible to control closely green ball firingconditions and to eliminate other serious shortcomings and derelictionsof methods and apparatus of the prior art devised for the production ofacceptable blast furnace pellets. Thus, this control may be essentiallyautomatic, but whether automatic or manual it can be exercisedthroughout the period of operation of the furnace without sacrifice ofproduct yield. Actually, in accordance with this invention, yields ofacceptable pellets will average consistently higher than the prior arthas been able to secure through the use of hematite orhematite-magnetite mixture, and the best firing methods and apparatusheretofore known. In other words, this invention enables, for the firsttime, the economic production on a commercial scale of pellets ofhematite of quality rivaling that of magnetite pellets.

Another important new advantage of the present invention concernsapparatus capacity, it being possible to fire with consistently goodresults green balls in bed depths twice to three times those recommendedin the prior art. In fact, from the standpoint of firingcharacteristics, there is theoretically no limit upon bed depth, but asa practical matter, the design considerations will prevent the buildingand operation of a furnace large enough to fire beds five to ten timesconventional depths.

Still another advantage of this invention is the heatexchange efficiencyobtainable in the ordinary practice of this invention, there beingeffective use of heat in the pellet bed during the combustion period andthere being in addition a feature of substantial heat recovery from theefiiuent gases.

A further advantage of this invention is that it provides a processwhich can be carried out continuously with wide variations in the natureof the green balls. Thus, the chemical characteristics of the balls mayvary from essentially pure hematite iron to essentially pure magnetiteiron Without either overfiring or underfiring the pellets and withouteither diminishing pellet yields or inordinately increasing the fuelcosts. Likewise, variations in the physical characteristics of thepellets will not affect the efficiency of the firing operation in theusual practice of this invention.

Still further, the use of solid fuel in the bed of green balls isunusually efficient in accordance with this invention, being graduatedaccording to the location of a given layer of green balls in the bed andaccording to the mass of each green ball. As an additional advantage, nosolid fuel is necessary outside the bed during the firing operation andonly in the brief period during which the base layer is heated at theoutset of the operation is it necessary to employ any external fuel tocarry out the process.

Another advantage of this process is that a portion of the product maybe clinkered where there is a particular use for coalesced induratedpellets. The remainder of the pellet product, however, will be in theform of discrete pellets at the time of discharge from the furnace ofthis invention, making separation of the clinkers simple and easy. Arelated advantage is that pellets of a wide variety of sizes for varyingrequirements may be made at one time in this apparatus.

A further important advantage of this invention is that the capacityexpressed as long tons per square foot per day can be unusually high by.prior art standards. Furthermore, while it was necessary in accordancewith the prior art traveling grate practice to reduce to relatively lowlevels the moisture content of green balls prior to firing them, this isnot essential in carrying out the operations of this invention.

By virtue of our invention, it is possible to use coke breeze or coarsecoal in its natural structure, by contrast with the prior art practiceswhich required special, finelydivided coal. However, we prefer to usefuels of the high fixed-carbon class of which anthracite coal isrepresentative.

The method of our invention, broadly described, comprises the steps ofconverting finely-divided, raw, iron-ore concentrates which consistmainly of hematite or magnetite into green balls, i.e. substantiallyspheroidal compacts which are relatively smooth and hard, establishingon a traveling grate a porous bed of incandescent solid material,distributing the green balls and solid fuel on the porous incandescentbed in overlying gas-pervious layers, flowing gases through said porousincandescent bed and upwardly through the overlying charge of greenballs and solid fuel, and thereby igniting and burning the said fuel andfiring the green balls.

In one of its apparatus aspects, the present invention, in general,comprises in a green ball-firing furnace having a traveling grate andmeans for delivering streams of gas into the green ball charge on thegrate, the combination of means for regulating the volume of the gasstreams and means for sensing the rate of combustion of the chargeoperatively associated with the gas delivery means to control the flowof gas into the green ball charge and to maintain the charge combustionrate within predetermined limits.

In another of its apparatus aspects, the present invention, as appliedto a green ball-firing furnace having a traveling grate and a pluralityof wind boxes along the grate travel course, comprises in general thecombination of heat recovery means including a hood for collecting gasesemerging from the charge at elevated temperatures and a ductcommunicating with the wind boxes and the hood for introducing collectedgases into the charge.

These methods and apparatus of this invention are described in detailbelow for the benefit of those skilled in the art, reference being hadto the drawings accompanying and forming a part of this specification,in which:

FIG. 1 is an end elevational view, partly in section, of pelletizingapparatus embodying this invention in assembly with green balls makingand rerolling apparatus and transfer means for delivering green ballsinto the pelletizing furnace;

FIG. 2 is a chart bearing a curve indicating the relationship betweenexhaust gas temperature and time or furnace length in typical operationof apparatus of this invention;

FIG. 3 is a chart bearing a curve representing the flow of air atdifferent times or locations in the furnace length bed resistance to airflow being constant;

FIG. 4 is a diagrammatic view of the apparatus of this inventionindicating the manner in which the green ball charge is delivered intothe traveling grate and the way in which the charge is treated inrespect to gas currents in the four zones of the furnace;

FIG. 5 is an enlarged diagrammatic view showing the structure of a greenball bed of this invention from the outset of the charging operationuntil the final layer is applied to the top of the bed;

FIG. 6 is another diagrammatic view similar to FIG. 5, but indicatingthe progress of the combustion zone upwardly through the bed of greenballs and pellets and showing the temperature variations as measured inthe hot gases approximately 12 inches above the bed;

FIG. 7 is an enlarged diagrammatic view indicating the structure of aspecial form of discharge from the furnace;

FIG. 8 is a diagrammatic view of apparatus of this inventionillustrating the collection of several efliueut gas fractions and therecirculation and the filtration of one of these fractions in accordancewith preferred practice;

FIG. 9 is a fragmentary, side elevational view of a pelletizing furnaceembodying this invention;

FIG. 10 is a fragmentary, section elevational view taken on line Iii-10of FIG. 9;

FIG. 11 is a schematic, side elevation of another pelletizing furnaceembodying this invention in a preferred form;

FIG. 12 is a diagrammatic view of the combustion zone portion of theFIG. 11 furnace, showing the location of the temperature-sensing meansand indicating the manner in which combustion is controlled; and

FIG. 13 is an enlarged, fragmentary, diagrammatic view of FIG. 11furnace equipped with an alternative type of combustion control system.

With reference to FIG. 1, a preferred arrangement of apparatus for thecommercial practice of our invention includes a balling device such as aballing disc 19 into which ore fines obtained from a froth floatationprocess or other commercial separation processes are introduced byconveyor 11 with an adequate amount of water suitably in the form of afine spray. Additives, such as limestone and bentonite clay infinely-divided form, are also introduced into the balling disc, all inaccordance with the teachings of our copending application Serial No.580,423, filed February 15, 1956, and application Serial No. 346,786,filed April 3, 1953. Green balls rolling from the lip of disc 10 arefuel-coated and ready for firing, having been rerolled in disc 19 withfinely-divided coal delivered into the reroll zone of the disc byconveyor 15. These rerolled green balls are carried by belt 20 from thedisc into pelletizing furnace 22. Depending upon the size of theoperation and the number of green ball layers to be established in thefiring bed, there may be two or more green ball lines of this type,preferably arranged in parallel relation.

As shown in FIGS. 1, 9, and 10, furnace 22 comprises an endless conveyor23 which is driven by a suitable motor means (not shown) through atravel course around cylinders 24 and 25 disposed outside the two endsof the furnace. Conveyor 23 has grate bars 27 which provide a floor forthe establishment of pellet beds and with a series of opposed side walls28 and 29 extending at right angles to the grate bars to support thepellet bed in its travel. In preferred form, the grate bars and sidewalls extend around the entire length of the conveyor, but the sidewalls are articulated in short length so that the grate can be freerunning in its travel around its closed course.

The travel course is defined by a pair of rails 35 which are supportedby a plurality of opposed beams 36 carried by columns 37 arranged in twoopposed series on opposite sides of the conveyor 23.

A hood 44) is disposed along the course of the traveling grate,enclosing that portion of the grate in the pelletfiring and coolingzones of the furnace. The hood has lower portions extending downwardlyon opposite sides of the side walls of the grate to a point adjacent tograte bars 27 and has observation ports 41 and access doors 42 atintervals along its length, as indicated in FIG. 9. The hoodcommunicates with a duct 43 which serves to carry off gases emergingfrom the pellet bed and hood and duct are provided with partitions 44and 45 (FIG. 8) which divide this gas-handling system into threeseparate zones 47, 48, and 49, each of which is provided with a separateoutlet for the collected gases. In zone 47, moisture-laden gases evolvedas a result of the combustion of the solid fuel in the pellet bed arecollected through the hood and duct assembly and exhausted through port50 and conduit 51. Middle zone 48 serves pellet bed at the time of itsto collect gases which are substantially free from combustion productsand moisture but are at elevated temperature, exhausting them throughopening 52 and a conduit 53 equipped with a filter 54. Conduit 53exhausts into gas-delivery line 55 serving the combustion zone of thefurnace and is connected to a source of air by means of a conduit 56.Fan 57 downstream from filter 54 in conduit 53 draws gases through thisfilter which serves to remove dirt from the gases and thereby protectthe impellers of fan 57 and also eliminate the danger of clogging thebed of wet green pellets in the ignition zone of the furnace. A fan 58is disposed in conduit 56 to supplement the output of fan 57 with freshair and it will be understood that when an excess of preheated air isdelivered by fan 57, conduit 56 may be used to vent the excess air tothe atmosphere. In the lower end of the furnace where the pellet bed issubjected to updraft cooling, as will be subsequent described, gasescollected in the hood and duct assembly are exhausted from the systemthrough the open end of the duct 43 by suitable stack means (not shown).

At the discharge end of the traveling grate, wind boxes 58 are providedas shown in FIG. 9, this being the downdraft cooling zone of thefurnace. Suitable connections to a source of air under pressure and atordinary temperatures are made into boxes 58 for the establishment ofdowndrafts through the bed.

Similarlly, at the other extremity of the furnace in the ignition zoneand just in advance of the first green ballcharging station, burnerboxes 60 are disposed over the conveyor for the application of gasflames or hot gases to solid material provided at this point on thegrate to create an incandescent bed for subsequent bed ignition, as willbe described. Ducts 62 and 63, respectively, are located under the upperlevel of conveyor 23 in the ignition zone and the downdraft cooling zoneto receive gases flowing from the grate downwardly at each of thesestages. These gases are exhausted from the system by any suitable means.

Through the length of the furnace between the ignition zone and thedischarge zone, a series of wind boxes 65 are disposed between the upperand lower levels of the traveling grate assembly. The upper end of eachof these boxes is open to the full width of the bed as determined by thespacing of side walls 28 and 29, while the lower end portion 66 of eachbox 65 is restricted for discharge through a relatively small openinginto a dustcollecting line 68, as illustrated in FIG. 10. Air isdelivered into 65 by a plurality of separate pipes 70 which are directedfor discharge of air upwardly through the upper end of these boxes andinto the traveling grate and the charge carried thereon. Blower means inthe form of a plurality of separate fans 72 (one shown) are provided tosupply regulated quantities of air at the required pressures for thefiring operation. The operation of fans 72 and the volume of airdelivered into the pellet bed traveling through the furnace may bemanually regulated, but in accordance with the preferred practice of thepresent invention, this is automatically accomplished by means of thetypes illustrated in FIGS. l2 and 13.

Furnace 77 of FIG. 11 is in general similar to furnace 22, being anupdraft traveling grate designed for continuous operation underautomatic control. Thus the driving, supporting and sealing means may beessentially the same in each instance and may be like the correspondingcomponents of conventional sintering machines. The air delivery and gasexhaust systems, however, are different in these two furnaces and ourpresent preference is the FIG. 11 arrangement.

Traveling grate 78 of furnace 77 extends over a course running fromright to left as seen in FIG. 11 along which are disposed severalstations or zones. Thus, fired pellets are loaded continuously on grate73 at the first station and ignition coal is deposited more or lessuniformly on the resulting burned pellet bed to be carried by the gratethrough ignition furnace 79 (ignition zone) wherein propane gas flamesburn the fuel and heat said pellets to incandescence. Green ball chargeis then deposited in four stages on the grate in the combustion zone ofthe furnace, induration being effected by the upward delivery of heatedair through the grates under conditions of automatic control asindicated in FIGS. 12 and 13 and described in detail below. Gasesemerging from the pellet bed in the combustion zone are collected inhood 80 and exhausted from the system.

When induration is complete, as indicated by the zone line in FIGS. 11,12, and 13, the cooling operation is begun by reason of the advancementof grate '78 carrying the indurated pellets into the cooling zone. Freshair is delivered into the grate charge at this station by means of awindbox duct 81 and windboxes 82 and the resulting heated air emergingfrom the top of the indurated charge is collected by hood 83 and thenconducted into the windbox duct serving the combustion zone windboxes, aconduit 34 being provided for this purpose and a filter 85 beingdisposed in the conduit to clean this heated air prior to itsintroduction into the combustion zone. The grate charge is next conveyedinto a downdraft cooling zone in which fresh air is delivered downwardlyinto the top of the indurated charge and exhausted from the systemthrough a duct disposed from the bottom of the grate, this air normallybeing of such low heating value that recuperation or heating recoveryfrom it is not feasible. The thus cooled indurated charge is nextcarried into the furnace discharge zone and dumped from the travelinggrate on to suitable conveyors or into shipping or storage vessels.

The combustion control system of FIG. 12 as applied to the combustionzone of furnace 77 comprises four thermopile units 86 located in hood 80at points substantially uniformly spaced above the respective green balllayers of the furnace charge bed. Four motors 74 for operating dampers87 of the windboxes are electrically conected to the respectivethermopiles through controller circuits 3%. The rate of delivery of airinto the combustion zone duct may be substantially constant or it may beregulated by suitable automatic control means according to the rate offlow of air from the windboxes into the traveling grate. Thus asillustrated in FIG. 13 blower fan 90 is controlled by thermopile 91 inhood 80 in cooperation with a controller circuit which regulates thespeed of fan motor 92.

The controller circuits which we prefer for use in these assemblies areexemplified by the Minneapolis Honeywell Electroline Control System (seeMinneapolis Honeywell Catalog No. 1531). Thus such a circuit includes anElectr-O-Line relay, Electronik proportional controllers, and anelectric proportional control motor. Thermopiles 86 are connected totheir respective proportional controllers so that when temperaturesensed by a thermopile varies from a predetermined temperature, thecircuit including that thermopile is lined-out and the said relay withits automatic reset feature will cause the final control element, i.e.the damper 37 controlled by this particular circuit to assume thecorrect position through the operation of its motor 74 to maintaincontrol in a predetermined temperature range.

The normal operations of these FIGS. 12 and 13 systems are indicated inFlGS. 2 and 3, wherein it is apparent that blowing rates are related tobed temperatures over the major part of the resident period of anincrement of furnace bed or charge. When combustion is completed theblowing rate is increased to effect rapid cooling of the charge inpreparation for removing the indurated pellets from the furnace. Anothercharacteristic of this operation is the reduction in blowing rate witheach successive charge, this being due to the tendency for bedresistance to build higher at deeper bed depth and to increase with eachcharge,

Operation of the FIG. 1 apparatus is carried out in a similar manner.Thus, prior to the delivery of the first green balls into the furnace, alayer 95 of suitable material, such as indurated pellets, solid fuel, orboth, is deposited on grate 23 and is brought to incandescence by directapplication of a flame or'by combustion of the solid fuel. With thegrate traveling continuously along its course at a predetermined rate,layer or bed 95, as it reaches temperature, is carried continuously intothe first green ball charging station. Subsequent charging operationsare consequently continuous and at fixed rate, but with the possibilitythat charging ratios may be altered at any time that it is desired tomodify the constituency of the bed as to pellet size, distribution orfuel form or ratio.

In the first green ball charging stage, balls prepared by the proceduredescribed above in the apparatus illustrated in FIG. 1 are deliveredonto the incandescent bed continuously and at a predetermined rate toestablish a primary layer. These balls may, as indicated above, be ofmagnetite or hematite or mixtures thereof and suitably compounded forphysical characteristics desired in the final burned pellets as well asin green ball form. Normally, it is preferable to charge relativelysmall diameter green balls into the furnace at the first station, butthe present invention contemplates the charging of green balls of anydesired or suitable size in this phase of the process, as well as insubsequent bed-building steps.

As shown in FIG. 5, as the grate moves along its course, a second chargeof green balls is made, forming a second layer of balls of slightlylarger size than those in the base layer. Further travel of the conveyorthrough the firing zone of the furnace involves the establishment ofsuccessive layers of balls of gradually increasing size until in theoperation illustrated a total of eight distinct layers have beenestablished on top of the original ignition layer 95.

Throughout the period that the grate is traveling through the successiveball-charging stations, an updraft of a mixture of ambient fresh air andfiltered, warm furnace air may be delivered into the lower part of thegrate and into the bed laying thereon in the form of a number ofseparate streams. Windboxes 65, fans 72 and pipes 70 connecting them tothe windboxes provide the air pressures required beneath the pellet bedto cause streams of air to flow through the grate to accomplish thefiring result required. FIG. 2 illustrates the condition as it may betypically established and maintained in the operation of a furnace ofthis type where the volume of the air delivered into the pellet bed asit travels along the furnace grate course is shown to increase atapproximately the point where firing of the bed is completed. Lowerblowing rates are employed while combustion is taking place in the bedthan subsequent to that time because of the danger of not firing the bedthoroughly at high blowing rates which strip heat from the pellet bed.

Referring to FIG. 6, a four-level bed is established in accordance withthis invention so as to take full advantage of the heat recoveriespossible in the bed and to use the successive pellet charges as meansfor controlling the temperatures within the bed. Stepwise, additions ofgreen balls along the course of the grate travel also provide theadvantages indicated above of maintaining uniform firing conditions inall portions of the bed so that beds of virtually any desired depthwithin the limits of machine design may be produced with satisfactoryindurated pellets resulting. As this diagram shows, combustion begins inthe bed almost as soon as the green balls are deposited at the firstcharging station on incandescent layer 95. The rate of combustion of thesolid fuel associated with the green balls as a coating or as anadmixture in the bed is indicated as being substantially a straight linefunction of the grate travel rate, but those skilled in the art willunderstand that satisfactory results may be obtained consistently whenthese ratios are varied. One of the important things is that conditionsas to moisture in the bed are controlled to prevent moshing of the greenballs of the upper layers due to cooling of gases rising through the bedand condensation of their moisture burden. Accordingly, when the firstlayer of green balls is burned close to the upper surface and theuppermost part of this bed shows a temperature of approximately 350" F.indicating elimination of moisture, a second layer of green balls isplaced upon the substantially fired first layer, dropping thetemperature of the surface to about 210 F. and indicating that the heatrising through the bed from the combustion zone is being used toevaporate the moisture from the green balls of the second charge. Infurther travel upwardly through the bed of the combustion zone and withfurther travel of the conveyor, the surface of the second charge zoneshows an increase to 350 F. with the elimination of the last of themoisture. A repetition of the charging operation to produce a thirdlayer of green balls brings the resulting surface temperature down toabout 230 F. in a typical operation with a subsequent rise to 350 F. asthe combustion zone again approaches the top of the bed; A fourthchanging stage has the same effects, but thereafter the bed is fired outas the conveyor continues along its course and no further change is madewith the result that eventually the combustion zone reaches the top ofthe last charge and the bed surface is at a temperature of 2500 F. Fromthis point onward, the effort is in the direction of cooling the chargein preparation for its removal from the furnace. The combustion zoneappears at the surface of the charge relatively early in the firing zoneand cooling of the bed takes place continuously substantially from thattime. Actually, updraft cooling is begun almost as soon as the chargehas burned through, and this is followed by a relatively short period ofdowndraft cooling to approximately room temperature.

All bed temperatures stated herein and indicated in the drawings areactually temperatures of the hot gases twelve inches above the portionof the bed specified.

When the combustion zone breaks through the top of the bed so that allof the fuel in the bed is exhausted and the gases emerging from the bedare free from both moisture and combustion products, recovery of thesensible heat in these gases is a practical possibility. Thus, inaccordance with this invention, gases of this type collected through themiddle portion of the length of the furnace, as indicated in FIG. 8, arewithdrawn from duct compartment 48 separately from the cooler gases ofcompartment W, and the contaminated gases in compartsent 47, andfiltered and reintroduced into the pellet bed in the combustion zone. Asis shown, it may be desirable to supplement these gases with fresh airas make up and this may be regulated through control over the operationof fan 58.

In charging green balls into the traveling grate as stated above, theymay be provided with coatings of sufiicient solid fuel to meet theirheating needs in the firing operation. Alternatively, they may beadmixed in the bed with solid fuel in the required amount, being chargedtogether therewith or being added separately to establish the desiredratio and bed constituency. Furthermore, by virtue of the heatrecoveries possible in accordance with this invention, the ratio of fuelmay be diminished in successive layers of the bed so that the ratio offuel to pellet bulk will vary from rich to lean from the bottom of thebed to the top of the pellet bed.

In accordance with our preferred practice of this invention, whenanthracite coal or similar high carboncontent solid fuel is employed andis added separately to the green ball bed, the amounts and particlessizes of this fuel are regulated so that fuel distribution through thebed is substantially uniform and each void in the bed is filled toapproximately one-third of its total volume.

Also, the green balls will be charged onto the grate so that betweenabout 35% and about 55% of the resulting bed volume, excepting theseparately added solid fuel, will represent voids. Thus the total voidvolume is diminished by the solid fuel by approximately one-third in ourusual practice, but it will be understood that satisfactory results canbe consistently obtained when the quantity of fuel is between aboutone-fifth and one-third of the total bed void volume, i.e. betweenapproximately one-fifteenth and one-sixth of the aggregate volume of thegreen ball bed. However, it is important that the bed he pervious togases and this requires that the amount of fuel be limited and that itsdistribution be such that no substantial proportion of the bed voids beeliminated with the result that the bed is effectively plugged.Additions of substantially more solid fuel than the limits stated abovemay be offset in their deleterious effects upon bed firing by increasedblowing rates but this remedy will normally involve an economicdisadvantage because of increased power requirements and possibly alsobecause of lost heat values of the fuel.

Having thus described this invention in such full, clear, concise andexact terms as to enable any person skilled in the art to which itpertains to make and use the same, and having set forth the best modecontemplated of carrying out this invention, we state that the subjectmatter which we regard as being our invention is particularly pointedout and distinctly claimed in what is claimed, it being understood thatequivalents or modifications of, or substitutions for, parts of theabove specifically described embodiments of the invention may be madewithout departing from the scope of the invention as set forth in Whatis claimed.

What is claimed is:

1. The method of agglomerating finely-divided, raw, iron-oreconcentrates in which the major portion of the iron is present in theform of a compound selected from the group consisting of hematite andmagnetite which comprises the steps of forming green balls of saidconcentrates of various sizes, distributing said green balls and solidfuel on a grate as a bed of graduated green ball size with the largestgreen balls at the top of the bed, and flowing gases at elevatedtemperatures upwardly through the said bed and thereby igniting andburning the said fuel and firing the green balls.

2. The method set forth in claim 1 in which the solid fuel is sufficientto fill about one-third of the voids between the green balls.

3. The method set forth in claim 1 in which the voids between the greenballs constitute between about 35% and about 55% of the total bed volumeand the solid fuel is sufiicient to fill about one-third of the totalvolume of the voids.

4. The method of agglomerating finely-divided, raw, iron-oreconcentrates in which the major portion of the iron is present in theform of a compound selected from the group consisting of hematite andmagnetite which comprises the steps of forming green balls of saidconcentrates of various sizes, distributing said green balls and solidfuel on a grate in a series of layers in which the green balls aregraduated as to size between layers, and flowing gases at elevatedtemperatures through the resulting bed from the smaller green ball-sizelayer to the larger green ball-size layer and thereby igniting andburning the said fuel and firing said green balls.

5. The method of agglomerating finely-divided, raw, iron-oreconcentrates in which the major portion of the iron is present in theform of a compound selected from the group consisting of hematite andmagnetite which comprises the steps of forming green balls of saidconcentrates of different sizes ranging between about A" and about /2 indiameter, forming a plurality of layers of said green balls andanthracite coal on a porous bed of incandescent solid material on agrate, the size of the balls being smallest in the lowermost layer andincreasing in size in the layers thereabove, flowing gases at elevatedtemperatures upwardly through the layers of green balls and said coaland thereby drying the green ball layers in sequence, and igniting andburning the said coal and firing the lowermost green ball layer beforeuppermost green ball layer has been dried.

6. The method of agglomerating finely-divided, raw, iron-oreconcentrates in which the major portion of the iron is present in theform of a compound selected from the group consisting of hematite andmagnetite which comprises the steps of forming green balls of saidconcentrates of various sizes between about /4" and about /2 indiameter, coating said balls with finely divided anthracite coal,forming a series of relatively shallow layers of said green balls, withthe size of the balls increasing from the bottom layer to the top layer,heating and removing substantially all the moisture from the green ballsof each layer in sequence, from bottom to top, and flowing gases atelevated temperatures upwardly through the resulting multi-layer bed ofgreen ball thereby firing the green balls in sequence according to theirlocation in the layered green ball bed so that as the lowermost layer isbeing fired the uppermost layer is being dried.

7. The method of agglomerating finely-divided, raw, iron-oreconcentrates in which the major portion of the iron is present in theform of a compound selected from the group consisting of hematite andmagnetite which comprise the steps of forming green balls of saidconcentrates of several difierent sizes, successively charging saidgreen balls into a furnace to establish a plurality of layers graduatedas to green ball size from a minimum of about /4" in diameter in thelowest layer to about /2" in diameter in the top layer, incorporatinghigh fixedcarbon content solid fuel in the bed in amount sufiicient tofire all the green balls, igniting the fuel in the lower portion of thefurnace charge, delivering a plurality of air streams upwardly into thebottom of the green ball bed and carrying the resulting combustion zoneupwardly through the successive layers of green balls, and regulatingthe volume and velocity of flow of the air streams to control the rateof combustion of fuel in the bed in such a manner that the moisture isdriven from the balls on one layer before another layer is appliedthereto.

8. The method of agglomerating finely-divided, raw, iron-oreconcentrates in which the major portion of the iron is in the form ofhematite or magnetite comprising thesteps of forming green balls of saidconcentrates of substantially uniform size between about inch and about/2 inch, providing on a grate a layer of indurated pellets, heating theindurated pellets to incandescence, providing a coating offinely-divided anthracite coal on said green balls, depositing thecoal-coated green balls on the incandescent pellets to a bed depth ofabout eight inches,

flowing air upwardly through the incandescent pellet layer and throughthe overlying green ball bed and igniting and burning the coal on thegreen balls, depositing additional anthracite coal-coated green balls toa bed depth of about eight inches on the first green ball bed when thezone of combustion approaches the top of the first said bed, continuingair flow upwardly through the grate charge, and depositing a third bedof anthracite coal-coated green balls of about eight inches depth whenthe zone of combustion has approached the top of the second green ballbed, and depositing a fourth anthracite coal-coated green ball bed abouteight inches deep on top of the third green ball bed when the combustionzone has approached the top of the third said bed, continuing the upwardflow of air through the grate charge until the combustion zone breaksthrough the top of the fourth bed, and then substantially increasing thevolume of air fiow through the bed to improve thermal efiiciency.

9. The method of making iron ore pellets suitable for use in a blastfurnace which comprises the steps of forming green balls by rollingfinely divided, moist, raw iron ore concentrates in which the majorportion of the iron is present as a compound selected from the groupconsisting of hematite and magnetite, forming a continuous layer ofindurated iron ore pellets on an endless, continuously traveling grate,heating pellets in said layer to incandescence by moving said layer pasta flame contacting the upper surface thereof, moving said layer into aclosed combustion chamber and depositing at least one layer of saidgreen balls and finely divided fuel on said incandescent pellets,flowing gases up through said pellets and green balls and burning saidfuel in said chamber, cooling the burned green balls in a closed coolingchamber by flowing gases up through the burned green balls, andcollecting gases in the cooling chamber ranging in temperature betweenabout 800 F. and 2500 F. and directing said gases upwardly through thegrate and materials thereon in the combustion chamber.

10. The method set forth in claim 9 in which a plurality of superimposedlayers of green balls and finely divided fuel are successively formed onsaid layer of pellets.

ll. The method of making iron ore pellets suitable for use in a blastfurnace which comprises the steps of forming green balls by rollingfinely divided, moist, raw iron ore concentrates in which the majorportion of the iron i present as a compound selected from the groupconsisting of hematite and magnetite and coating said green balls withfinely divided solid fuel, forming a continuous layer of indurated ironore pellets on an endless, continuously traveling grate, heating thepellets in the upper portion of said layer to incandescence by movingsaid pellets past a flame contacting the upper surface thereof, movingthe thus heated pellets into a closed combustion chamber and depositingat least one layer of said fuel coated green balls on said incandescentpellets, flowing gases up through said pellets and balls and burning thefuel on said green balls in said chamber, cooling the thus burned greenballs in a closed cooling chamber by flowing gas up through the burnedgreen balls, collecting gases from the cooling chamber ranging intemperature between about 800 F. and 2500 F., adding fresh air theretoand directing said mixed gases upwardly through the grate and materialsthereon in the combustion chamber.

12. The method of making iron ore pellets suitable for use in a blastfurnace which comprises the steps of forming green balls by rollingfinely divided, moist, raw iron ore concentrates in which the majorportion of the iron is present as a compound selected from the groupconsisting of hematite and magnetite, forming a continuous layer ofindurated iron ore pellets on an endless, continuously traveling grate,heating pellets in the upper portion of said layer to incandescence bymoving said layer past a flame contacting the upper surface thereof,moving said layer into a closed chamber having first, second and thirdzones, depositing at least one layer of said green balls and finelydivided fuel on said incandescent pellets, flowing gases up through saidpellets and green balls and igniting said fuel on said green balls insaid first zone. removing and discarding moisture ladened gases fromsaid first zone, burning said fuel and passing gases up through thegreen ball layer in the second zone, cooling the burned green balls inthe third zone by flowing air up through the burned green balls, andcollecting gases from the third zone ranging in temperature betweenabout 800 F. and 2500 F. and directing said gases upwardly through thegrate and materials thereon in the first and second zones.

13. In a pellet firing furnace the combination of an endless travelinggrate, means forming a closed chamber about and along the grate andhaving first, second and third zones, conduit means to remove moistureladened gases from above the grate in said first zone, conduit means toremove highly heated gases from above the grate in the said second zoneand discharge them into the first said zone below the grate, means tointroduce fresh air into said highly heated gases just prior to theirentry into said first zone, means to admit fresh air into said secondand third zones beneath said grate, and means including temperaturesensitive means in the upper part of said first zone for regulating thevolume of gases discharged into said first zone.

References Cited in the file of this patent UNITED STATES PATENTS DwightMar. 23, 1909 Heilman Oct. 1, 1918 Richards Jan. 21, 1919 Holm bergSept. 9, 1924 Lellep June 12, 1956 Davis Jan. 28, 1958 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,024,101 March 61962 Louis J. Erck et all,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 6, line 28, for "Similarlly" read Similarly line 4O for "Through"read Throughout, column 14, line 5 for "1,280,281" read 280,221 7-.

Signed and sealed this 3rd day of July 1962.

(SEAL) Attest:

ERNEST w. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

1. THE METHOD OF AGGLOMERATING FINELY-DIVIDED, RAW, IRON-ORECONCENTRATES IN WHICH THE MAJOR PORTION OF THE IRON IS PRESENT IN THEFORM OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF HEMATITE ANDMAGNETITE WHICH COMPRISES THE STEPS OF FORMING GREEN BALLS OF SAIDCONCENTRATES OF VARIOUS SIZES, DISTRIBUTING SAID GREEN BALLS AND SOLIDFUEL ON A GRATE AS A BED OF GRADUATED GREEN BALL SIZE WITH THE LARGESTGREEN BALLS AT THE TOP OF THE BED, AND FLOWING GASES AT ELEVATEDTEMPERATURES UPWARDLY THROUGH THE SAID BED AND THEREBY IGNITING ANDBURNING THE SAID FUEL AND FIRING THE GREEN BALLS.